This invention relates to an impedance-matching radio wave absorber, and more specifically, to a thin-layered radio wave absorber useful for absorption of high frequency electromagnetic waves.
Recently, with a demand for higher frequency of a signal used with electronic equipment, a problem of useless radiation emitted from the electronic equipment becomes marked. Control of suppressing the useless radiation from the electronic equipment may be made by a method for making a change to circuit designs, or employing anti-useless radiation components and so on. However, use of these methods becomes more and more difficult for reasons of a demand for shorter period of product span and an increase in cost or the like. In this connection, there is a tendency toward use of a method for applying an anti-useless radiation sheet or the like made up of a sheet-shaped composite soft magnetic material having a magnetic loss even for high frequency electromagnetic waves.
A wireless LAN (Local Area Network), a superhighway automatic accounting system or like communication systems making use of high frequency radio waves have been also recently developed. However, in a radio wave-handling equipment applied to these communication systems, any radio wave other than a target signal wave results in radio disturbance, so that development of a radio wave absorber has been required for smooth communication by absorption of the generated radio disturbance. Electromagnetic wave in the frequency band of 2.45 GHz, for instance, is used with various kinds of electronic equipment such as an electronic oven, a portable information terminal, a wireless LAN and a Bluetooth, and smooth communication when using these pieces of electronic equipment without mutual malfunctions is of importance.
While the radio wave absorber is useful to transform energy of incident radio wave into heat for absorption, it is supposed that a loss term ∈xe2x80x3 (an imaginary component of a complex relative permittivity) of a relative permittivity of the radio wave absorber and a loss term xcexcxe2x80x3 (an imaginary component of a complex relative magnetic permeability) of a relative magnetic permeability are related to transformation of the energy of incident radio wave. When incidence of a radio wave on a material having a loss as described above occurs, the energy of the radio wave is transformed into heat for absorption according to the following expression (1).
P=xc2xdxcfx89∈0∈xe2x80x3|E2|+xc2xdxcfx89xcexc0xcexcxe2x80x3|H2|xe2x80x83xe2x80x83(1) 
In the above expression (1), P represents wave absorption energy [W/m3] per unit volume, xcfx89 is the angular frequency (2xcfx80f, f: frequency of electromagnetic wave) of an electromagnetic wave, ∈0 is magnetic permeability of free space, ∈xe2x80x3 is an imaginary component of a complex relative permittivity (a dielectric loss), E is electric field strength of an electromagnetic wave applied from the outside, xcexc0 is magnetic permeability of free space, xcexcxe2x80x3 is an imaginary component of a complex relative magnetic permeability (a magnetic loss), and H is magnetic field strength of the electromagnetic wave applied from the outside.
According to the above expression (1), the higher loss that a material has, the greater will be the radio wave absorptive power. However, in a case of a plane wave in a relatively remote electromagnetic field at a distance of not less than xcex/6 (xcex: wavelength of an electromagnetic wave) from a wave source, if incidence of a radio wave on such a high loss material just for once is all that occurs, complete absorption of energy of the radio wave for transformation into heat is made impossible in most cases. This is because reflection takes place on a front face of the radio wave absorber for reason of a difference in impedance between air and the radio wave absorber.
Accordingly, in the radio wave absorber for absorption of a plane wave, a back face of the radio wave absorber is backed with a conductor, and absorption of the radio wave is made by a method for controlling phase of a reflected wave in an interface between the conductor and the radio wave absorber and a reflected wave in the front face of the radio wave absorber to offset the reflected waves each other. The wave absorber implemented by taking the above method is called an impedance-matching wave absorber. The impedance-matching radio wave absorber normally aims at a return loss of 20 dB, which is considered to be equivalent to a value representing 99% absorption of the energy of the radio wave, in most cases.
It is necessary for the impedance-matching radio wave absorber used for the high frequency band of not less than 1 GHz to have high relative magnetic permeability and high electric resistance. Conventionally, rubber ferrite, for instance, has been heretofore widely used as a material of the impedance-matching radio wave absorber. Otherwise, carbonyl iron, form styrol carbon or the like has been also in use. In the impedance-matching radio wave absorber, a matching frequency and a matching thickness are determined once a material constant is established. A thickness of about 1 cm, when using rubber ferrite or the like, is required for the electromagnetic wave in the frequency band of 2.45 GHz, resulting in use of the radio wave absorber having the above thickness in the conventional technique. However, with the progress of miniaturization of electronic equipment such as the portable information terminal, for instance, there is a need for smaller thickness of the radio wave absorber to reduce the proportion of a radio wave absorber size to an equipment size. In this connection, development of a radio wave absorber, which meets demands for smaller thickness and lighter weight while keeping up radio wave absorption performance with the use of a material of higher relative magnetic permeability, has been desired.
On the other hand, a thin film material containing Co is known as a material having high relative magnetic permeability enough to cover the high frequency band, as disclosed in Japanese Patent Application Laid-open No. 10-241938, for instance. Using this thin film material meets both high magnetic permeability and high electric resistance in a Coxe2x80x94Nixe2x80x94Alxe2x80x94O thin film or the like by adopting a granular structure composed of two or more kinds of fine structures such as fine magnetic particles limited in particle size to about 4 to 7 nm and grain boundaries of extremely thin ceramic film surrounding the fine magnetic particles. However, the thin film material in this case is formed in the shape of a thin film using a sputtering device, resulting in no application to a material of practical use as the radio wave absorber.
A multi-layered radio wave absorber including a magnetic layer consisting of the above material is also often applied as the impedance-matching radio wave absorber. The structure available may be that having a dielectric layer on the front face of a magnetic layer backed with the conductor as described above and so on, for instance. As compared with a single-layered radio wave absorber, the multi-layered wave absorber has advantages of easily managing matching of a reflected wave phase by reason that reflection is subjected to the control as impedance of an incident face nears space impedance, whereas having disadvantages of increasing the production cost. For that reason, in producing the impedance-matching radio wave absorber, there is a need for selection of a material and a structure in consideration of the above advantages and disadvantages, while difficulty has been experienced in passing decision on selection of the material and the structure.
The present invention is provided in view of the above circumstances, and its object is to provide a thinner-layered radio wave absorber, which permits exact selection of a material and a structure and achieves high absorption performance for high frequency electromagnetic wave.
According to the present invention, in order to solve the above problems, there is provided, in an impedance-matching radio wave absorber, a radio wave absorber comprising a magnetic layer having a thickness of not more than 1 mm and arranged to have values of a real part xcexcxe2x80x2 and an imaginary part xcexcxe2x80x3 of complex relative magnetic permeability satisfying the expression of xcexcxe2x80x3xe2x89xa7mxcexcxe2x80x2xe2x88x92n (m: real number of m greater than 0, n: real number of nxe2x89xa70) outside an impedance mismatching region, and a conductor fixedly attached to a face opposite to an electromagnetic-wave incident face of the magnetic layer.
The above radio wave absorber, even when having the magnetic layer of not more than 1 mm in thickness, achieves satisfactory absorption characteristics for high frequency electromagnetic wave by adopting the structure that the conductor is fixedly attached to the face opposite to the electromagnetic-wave incident face of the magnetic layer of single-layered structure, and arranging the magnetic layer to have the values of the real part xcexcxe2x80x2 and the imaginary part xcexcxe2x80x3 of the complex relative magnetic permeability of the magnetic layer satisfying the expression of xcexcxe2x80x3xe2x89xa7mxcexcxe2x80x2xe2x88x92n (m: real number of m greater than 0, n: real number of nxe2x89xa70) outside the impedance mismatching region. When the relative permittivity of the magnetic layer is not more than 15, the return loss of not less than 20 dB is achieved for the electromagnetic wave in the frequency band of 2.4 to 2.5 GHz, for instance, on the assumption that 4xe2x89xa6mxe2x89xa66 and nxe2x89xa630, while the return loss of not less than 10 dB is achieved on the assumption that 1.2xe2x89xa6mxe2x89xa61.5 and nxe2x89xa610. When the relative permittivity of the magnetic layer is not more than 50, the return loss of not less than 20 dB is also achieved on the assumption that 4xe2x89xa6mxe2x89xa66 and nxe2x89xa6100, while the return loss of not less than 10 dB is achieved on the assumption that 1.2xe2x89xa6mxe2x89xa6-1.5 and nxe2x89xa630. Using a magnetic material of fine textural structure limited in particle size to 1 to 100 nm in the shape of powder, for instance, for dispersion into a polymeric material or the like permits formation of the magnetic layer.
According to the present invention, there is also provided, in an impedance-matching radio wave absorber, a radio wave absorber, which comprises a radio wave absorptive layer having a thickness of not more than 1 mm and adopting a multi-layered structure including a magnetic layer arranged to have values of a real part xcexcxe2x80x2 and an imaginary part xcexcxe2x80x3 of complex relative magnetic permeability satisfying the expression of xcexcxe2x80x3xe2x89xa6mxcexcxe2x80x2xe2x88x92n (m: real number of m greater than 0, n: real number of nxe2x89xa70), and a conductor fixedly attached to a face opposite to an electromagnetic-wave incident face of the radio wave absorptive layer.
The above radio wave absorber, even when having the magnetic layer of not more than 1 mm in thickness, achieves satisfactory absorption characteristics for high frequency electromagnetic wave by adopting the structure that the conductor is fixedly attached to the face opposite to the electromagnetic-wave incident face of the radio wave absorptive layer including the magnetic layer, and also arranging the magnetic layer to have the values of the real part xcexcxe2x80x2 and the imaginary part xcexcxe2x80x3 of the complex magnetic permeability of the magnetic layer satisfying the expression of xcexcxe2x80x3xe2x89xa6mxcexcxe2x80x2xe2x88x92n (m: real number of m greater than 0, n: real number of nxe2x89xa70). When the relative permittivity of the magnetic layer is not more than 15, the return loss of not less than 20 dB is achieved for the electromagnetic wave in the frequency band of 2.4 to 2.5 GHz, for instance, on the assumption that 4xe2x89xa6mxe2x89xa66 and nxe2x89xa630, while the return loss of not less than 10 dB is achieved on the assumption that 1.2xe2x89xa6mxe2x89xa61.5 and nxe2x89xa610. When the relative permittivity of the magnetic layer is not more than 50, the return loss of not less than 20 dB is also achieved on the assumption that 4xe2x89xa6mxe2x89xa66 and nxe2x89xa6100, while the return loss of not less than 10 dB is achieved on the assumption that 1.2xe2x89xa6mxe2x89xa61.5 and nxe2x89xa630. Using a magnetic material of the fine textural structure limited in particle size to 1 to 100 nm in the shape of powder, for instance, for dispersion into a polymeric material or the like permits formation of the magnetic layer. The radio wave absorptive layer has a dielectric layer formed by kneading of ceramics with a polymeric material, for instance, in addition to the magnetic layer.